This invention relates to the recovery of hydrocarbons from an underground reservoir by cyclic injection and production of a recovery fluid comprising carbon dioxide or nitrogen, wherein the formation is subject to an active waterflood or water drive.
Numerous methods of enhanced oil recovery exist which involve the injection of a gaseous or a gaseous/liquid fluid into an underground formation. Recovery is often best where the fluid is injected at conditions so as to make the fluid miscible or conditionally miscible with the underground hydrocarbons. For non-thermal systems, the chief recovery fluid has been carbon dioxide.
Gaseous or gaseous/liquid recovery fluid methods may be divided into two types: drive processes and cyclic processes, which are also known as huff-n-puff or push/pull. In drive oil recovery processes, injection and production of fluids occur at different wells. In cyclic oil recovery processes, injection and production of fluids occur through the same well. Besides those structural differences, drive and cyclic processes are substantially different in that slugs of recovery fluid are designed differently, times of recovery are different, well patterns are different, costs are different, fluid velocities are different, and so forth.
Unlike drive recovery methods, cyclic processes are better suited for small oil reservoirs, particularly with the use of existing wells. The cost of recovery with a drive process in some smaller reservoirs, especially in deeper zones, may be so high as to make the reservoirs border-line candidates at best for oil recovery after primary or secondary recovery.
One of the earliest disclosures of a cyclic oil recovery process was in U.S. Pat. No. 3,480,081, wherein the flooding medium was water, brine or steam. The success of steam cyclic recovery processes inevitably lead to the cyclic injection and production of carbon dioxide with a soaking. U.S. Pat. No. 4,390,068 discloses such a carbon dioxide cyclic process. Cyclic carbon dioxide recovery has now become a commonplace event in the oil field.
Attempts to recover heavy oils and hydrocarbons from tar sands have lead to a number of processes involving the injection of various solvents and hot fluids in "pressurization and drawdown" methods. These are similar to cyclic carbon dioxide methods in that various solvents and fluids are injected into the formation through a well to increase formation pressure. The fluids may or may not be allowed to soak in the formation prior to producing the injected fluids along with hydrocarbons through the same well. U.S. Pat. No. 4,324,291 is one example of these processes.
The cyclic or huff-n-puff carbon dioxide process as described and practiced in the prior art is a immiscible process due to the reservoir conditions in which it is applied and because it is considered undesirable to push the hydrocarbons further away from the well on which the cyclic process is being applied. The process invariably is applied to pressure depleted reservoirs as a tertiary recovery step after the reservoir has been waterflooded out. These are pressure depleted reservoirs.
The typical cyclic carbon dioxide application relies on the injection of carbon dioxide in an immiscible condition to the hydrocarbons. Injection displaces a large proportion of the water phase within the wellbore vicinity, while bypassing the residual oil-in-place. Although the carbon dioxide absorbs into both the oil and remaining water, absorption into oil is a much slower process than absorption into water. For this reason the well is shut-in for what is termed a soak period. During the soak period, the oil will experience swelling, viscosity and interfacial tension reduction, and an increase in its relative mobility. When the well is returned to production, a portion of the mobilized oil is swept to the wellbore and produced. The prior art has considered it undesirable to perform a cyclic carbon dioxide process under miscible conditions since injection would push the hydrocarbons farther away from the wellbore, and render the hydrocarbons less likely to be recovered during the production phase.